Make LOCAL_SOURCE the default when running cros_chrome_make.
Make LOCAL_SOURCE the default when running cros_chrome_make.

A similar change was made to the ebuild in
https://gerrit.chromium.org/gerrit/#/c/34157/, but not for cros_chrome_make.
This simplifies the logic for determining CHROME_ORIGIN.  To use GERRIT_SOURCE,
users will need to explicitly set CHROME_ORIGIN.

BUG=chromium-os:34674
TEST=Verified cros_chrome_make defaults to LOCAL_SOURCE unless otherwise
     specified.

Change-Id: I1690ea725fe82671784ff13690d1daf177fb52db
Reviewed-on: https://gerrit.chromium.org/gerrit/35878
Reviewed-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Commit-Ready: Ryan Cui <5ce1592d53754a87a44fc2eb7b63d5f77cf50fdb@chromium.org>
Tested-by: Ryan Cui <5ce1592d53754a87a44fc2eb7b63d5f77cf50fdb@chromium.org>

coreos/dev-util,coreos/dev-util,coreos/dev-util,mgireesh05/dev-util,marineam/coreos-dev-util,mgireesh05/dev-util,marineam/coreos-dev-util,mgireesh05/dev-util